How to size a market you've never set foot in

The first market-size figure you find is usually too big. It was made to sell something: a report, a funding round, a strategy deck. When you are walking into a market you have never set foot in, the useful question is not "what is the number". It is "what has to be true for that number to hold".

Three routes to the same answer

We size unfamiliar markets three ways, and we keep the routes apart until the end.

• Top-down. Start from the published total and cut. Strip out the countries you will never serve, the buyers who will never switch, the price points your product has never sold at. Every cut gets argued for, not waved through.
• Bottom-up. Count real buyers. Hospitals, licence holders, checkout tills, sites with planning permission. Multiply by a price someone has actually paid and a share you could actually win. Small guesses compound fast here, so each one needs a source under it.
• Analogue. Find a market a few years further down the same road. Another country, a neighbouring category. Lay its history over your local conditions and mark the places where the map tears.

Each route fails in its own way. Top-down inherits someone else's boundary. Bottom-up compounds guesses. Analogues drift the moment regulation or culture differs in a way that turns out to matter. Running all three is not belt and braces. It is how the failure modes cancel out.

The disagreement is the finding

When the three routes land close together, you have your number. When they land a multiple apart, and in unfamiliar markets they often do, that gap is the most valuable thing the work has produced. Somewhere inside it sits an assumption that needs testing before anyone writes a cheque. Averaging the routes buries the question. Reconciling them answers it.

What lands on the client's desk is a range with named sensitivities: which assumption moves the answer most, what evidence would firm it up, and what to watch in year one to see which end of the range is arriving.

That is the difference between a number for a slide and a number for a decision. Someone takes the second kind apart, and it still stands.